1. Field of the Invention
This invention relates to an image processing method and a computer readable medium for image processing for visualizing a tubular tissue.
2. Description of the Related Art
A technique for visualizing the inside of a three-dimensional object has attracted public attention with the advance of image processing technology using a computer in recent years. Particularly in the medical field, medical diagnosis using a CT (Computed Tomography) apparatus or MRI (Magnetic Resonance Imaging) apparatus has been performed widely because a lesion can be detected early by visualizing the inside of a living body.
On the other hand, volume rendering is known as a method for obtaining a three-dimensional image of the inside of an object. In volume rendering, ray is emitted onto a three-dimensional voxel (micro volume element) space to there by project an image on a projection plane. This operation is referred to as ray casing. In ray casting, a voxel value is acquired from a voxel at each sampling point which is sampled at a regular interval along the path of the ray.
The voxel is a unit for constituting a three-dimensional region of an object. The voxel value is a specific data expressing characteristic such as a density value of the voxel. The whole object is expressed by voxel data which is a three-dimensional arrangement of the voxel value. Generally, two-dimensional tomogram data obtained by CT is collected along a direction perpendicular to each sectional layer, and voxel data which is the three-dimensional arrangement of voxel value is obtained by performing necessary interpolation.
In ray casting, reflected light of a virtual ray emitted onto an object from a viewpoint is generated according to an opacity value artificially set for each voxel value. Then, the gradient of voxel data, that is, a normal vector is obtained to obtain a virtual surface, and a shading coefficient for shading is calculated from the cosine of an angle between the virtual ray and the normal vector. Reflected light is calculated by multiplying the intensity of the virtual ray emitted on each voxel, the opacity value of the voxel and the shading coefficient.
FIG. 28A shows an example of a colon being displayed by a parallel projection method of volume rendering as an example of visualization of a tubular tissue in the inside of a human body. According to such volume rendering, a fluoroscopic image of the three-dimensional structure of the colon can be formed from two-dimensional tomogram data obtained successively along a direction perpendicular to sectional layers of the abdomen. The image obtained by the parallel projection method is suitable for observation from the outside but unsuitable for observation from the inside.
FIG. 28B shows an example of achieving an image obtained by a virtual endoscope by generating a centrally projected image of the inside of the colon with volume rendering. When voxel data is reconstructed from a viewpoint in the inside of the tubular tissue in this manner, inspection with an endoscope can be simulated. Accordingly, a polyp or the like in the inside of the tubular tissue can be detected. However, the virtual endoscope image has a disadvantage that a large number of images obtained by the virtual endoscope has to be referred to perform diagnosis because the region allowed to be displayed at one time in each image obtained by the virtual endoscope is small.
FIGS. 29A and 29B are views for explaining a parallel projection method and a central projection method respectively In the parallel projection method, as shown in FIG. 29A, virtual ray 82 is emitted parallel from a viewpoint 81, and an image can be generated to observe an observation target 83 mainly from the outside. On the other hand, in the central projection method, as shown in FIG. 29B, virtual ray 85 is emitted radially from a viewpoint 84. In the central projection method, an image with perspective and reality as the human sees an observation target 86 with his eyes can be generated.
FIGS. 30A and 30B show an example of display of an exfoliated image of a tubular tissue using a cylindrical coordinate system in ray casting. According to the central projection method shown in FIG. 29B, inspection of the colon or the like with an endoscope can be simulated, but it is difficult to understand the position or size of a polyp or the like in the wall of the tubular tissue accurately when the inside of the colon is inspected while scanned.
Therefore, as shown in FIG. 30A, a viewpoint 91 is placed on a center line 94 of a colon 93. Virtual ray 92 is radiated from the viewpoint 91 in directions perpendicular to the center line 94, and an image of the inner wall surface of the colon 93 is generated. Then, the image is cut open in parallel to the center line 94 so that an exfoliated image of the inner wall surface of the colon can be displayed as shown in FIG. 30B.
FIGS. 31A to 31E are views for explaining a cylindrical projection method using a cylindrical coordinate system. FIG. 31A shows a cylindrical coordinate system 102 set in the inside of a tubular tissue 101 and a virtual ray 103 radiated from the center axis of the cylindrical coordinate system 102. FIG. 31B shows a state in which the cylindrical coordinate system 102 is represented as C(h,α) based on a distance h along the center axis and an angle α around the center axis. FIG. 31C shows a state in which the cylindrical coordinate C(h,α) is exfoliated and converted into two-dimensional coordinates l(u,v) Each of FIGS. 31D and 31E shows a state in which the virtual ray 103 is radiated from the center axis of the tubular tissue 101. Accordingly, by assuming that a cylindrical coordinate system 102 is set virtually in the inside of a tubular tissue 101 and performing the projection radially from the center axis of the cylindrical coordinate system 102 in this manner, a 360° panoramic image of the inner wall surface of the tubular tissue 101 can be generated.
FIGS. 32A and 32B are views for explaining a curved cylindrical projection method when a tubular tissue as a observation object is curved. As shown in FIGS. 32A and 32B, the curved cylindrical projection method is a method of projection in which virtual ray 113 is radiated from a curved center line 112 when the tubular tissue 111 as a observation object is curved. As described above, in accordance with the curved cylindrical projection method, by assuming the central path 112 along the real curved internal organ of the human body, and by performing projection with the central path 112 as the center, virtual endoscopy inspection can be performed with CT data.
FIG. 33 is a flowchart of a curved cylindrical projection method in a related art. In the curved cylindrical projection method in the related art, first a central path is set (step S11) and a position t on the central path is initialized to t=0 (step S12).
Next, coordinates P (x, y, z) of the position t on the central path and a direction vector D (x, y, z) of the center path of the position t on the central path are acquired (step S13) Virtual rays are projected 360° on a plane, which is perpendicular to D (x, y, z) from P (x, y, z) (step S14).
Next, t is incremented (step S15), and to determine whether or not the end point position is reached, a comparison is made between the values of t and t_max. If t is smaller than t_max (YES), the process returns to step S12; if t is equal to or greater than t_max (NO), the processing is completed.
Thus, the curved cylindrical projection method in the related art becomes the same as the cylindrical projection method in the related art wherein every ray projected from a point on one central path lies along a plane having D (x, y, z) as a normal vector.
A method of projecting virtual rays so as to follow a virtual magnetic curve created from a path (for example, refer to U.S. Pat. No. 6,212,420), and a method of expanding an observation object using a finite element method before conducting cylindrical projection (for example, refer to “Virtual Colon Unfolding,” A. Vilanova Bartroli, R. Wegenkittl, A. Konig, E. Groller, IEEE Visualization, USA, 2001, p 411-420) are known as related arts.
FIG. 34 is a drawing to describe a problem of the curved cylindrical projection method in the related art. In the curved cylindrical projection method in the related art, virtual rays 118 to 127 are projected perpendicularly from a central path 112 (the normal vector of the plane along which the virtual rays lie is the same direction as the central path 112) and thus virtual rays 123 and 124 and virtual rays 125 and 126 cross each other in a large bend portion B of the central path 112.
If virtual rays 118, 119, 120, 121, etc., quiver in subordination to a meandering of the central path 112, then this results in an image difficult to grasp the state of a large intestine 111. That is, in a straight portion A of the large intestine 111, the virtual rays faithfully quiver in subordination to a meandering of the central path 112 and scales 128 and 129 of the image effected and thus the representation of a physical length changes according to the position on the image along the central path 112. Since the virtual rays cross each other in the large bend portion B of the central path 112, the same observation object 130 is duplicately displayed in the crossing part.
In “Virtual Colon Unfolding”, A. Vilanova Bartroli, R. Wegenkittl, A. Konig, E. Groller, IEEE Visualization, USA, 2001, p 411-420, above problem is tried to be solved by a method in which a folded structure of a surface of the target internal organ is unfolded by an approach of finite-element deformation after obtaining the shape of the surface of the target internal organ. However, it is difficult to say that this method is practical, because this method has disadvantages such as that subjective and complex condition setting is necessary in the extraction of the surface of the internal organ, and in the process of unfolding, lesion can not be detected because polyp is also unfolded, and calculation for extracting and unfolding the surface of the internal organ is enormous. Further, in U.S. Pat. No. 6,212,420 using the virtual magnetic curve, there exists an enormous load for calculating the magnetic curve.